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These data strongly suggest that the difference between the results of the two programs is caused by the different sampling techniques used. More recent studies in which simultaneous impactor and filter samples were taken showed (Cadle et ai, 1973) that a sizable part of the sulfate was really in the Aitken size range. However, we cannot rule out the possibility that the concentration differences may be explained partly by an increase of stratospheric sulfate burden arising from the volcanic activity between 1960 and 1970.

The first stratospheric aerosol chemical analyses showed that only a small quantity of the aerosol particles in the lower stratosphere could be of meteoritic origin (e.g. no nickel was found in the samples, see Table 24). This problem was studied in detail by Shedlovsky and Paisly (1966) who found by means of neutron activation of aerosol particles collected on filters that the stratospheric Fe/Na ratio is close to that reported for the Earth's crust. They concluded that less than 10 % of the iron and sodium identified at altitudes of 19-21 km come from meteorites. However, it is possible that between 30 and 50 km, where no such measurements were made, the meteoritic fraction of the aerosol is much more significant.

Junge (1963) speculated that the sulfate particles in the stratosphere are formed by chemical transformation of S02 and H2S gases mixing into the stratosphere mostly over the tropics. It was also reported that sulfur gases are injected into the stratosphere by volcanic eruptions (SMIC, 1971). According to Cadle and Powers (1966) the chemical conversion of sulfur gases above the tropopause is promoted by the presence of atomic oxygen, while more recent studies (see Subsection 3.6.3) suggest that OH radicals play an important role in the transformation of sulfur gases at these altitudes. Furthermore, Crutzen (1976) recently argued that biogenic carbonyl sulfide may be at least partially responsible for the formation of stratospheric sulfate layer. The sulfuric acid vapour produced by chemical reactions can condense homogeneously to form acid droplets diluted with water. However, there is some indication that this condensation takes place on existing Aitken-size particles. Thus, Mossop(1965) found that stratospheric solution droplets frequently contain small insoluble particles. Theoretical work by Hamill et al. (1977) also indicates that heterogeneous condensation of H2S04 and H20 vapours in the stratosphere is much more probable than a homogeneous phase transition.

Table 25

Mass concentration of sulfate and ammonium particles sampled at an altitude of 18 km on May 11. 1970 (Lazrus et al., 1971)

Table 25

Mass concentration of sulfate and ammonium particles sampled at an altitude of 18 km on May 11. 1970 (Lazrus et al., 1971)

Lutilude

Longitude

soi

nh;

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